User terminal

ABSTRACT

A user equipment for a mobile communication system, and method and chip thereof, receives a first parameter and a second parameter broadcasted from the current serving cell, and reselects a target cell to be used as a serving cell from among plural cells operated at different frequencies. The first parameter defines a probability for performing reselection of the target cell, and the second parameter is a timer value to be set in a timer started at a timing at which a quality measurement for neighbor cells ends. Reselecting includes, after receiving the first and second parameter, periodically measuring qualities of neighbor cells at timer defined timings, comparing a value corresponding to IMSI of the user equipment with a value corresponding to the first parameter, and reselecting the target cell from among cells having a quality satisfying predetermined quality criteria, based on results of the quality measurement and the comparison.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.15/727,450 filed Oct. 6, 2017, which is a Continuation of InternationalApplication No. PCT/JP2016/061617 filed Apr. 8, 2016, which claims thebenefit of U.S. Provisional Application No. 62/145,882 filed Apr. 10,2015, U.S. Provisional Application No. 62/148,967 filed Apr. 17, 2015,U.S. Provisional Application No. 62/162,204 filed May 15, 2015, and U.S.Provisional Application No. 62/188,867 filed Jul. 6, 2015, the entirecontents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a user terminal which is capable ofselecting a target cell to be used as a serving cell from among aplurality of cells operated at different frequencies.

BACKGROUND ART

In 3rd Generation Partnership Project (3GPP) which is a mobilecommunication system standardization project, a technique of selecting atarget cell to be used as a serving cell from among a plurality of cellsoperated at different frequencies has been proposed.

Specifically, in a case in which a start condition is satisfied, a userterminal measures qualities of neighbor cells adjacent to a currentserving cell and selects a target cell to be used as a serving cell fromamong cells satisfying a selection condition.

SUMMARY

A user equipment according to the present disclosure for a mobilecommunication system comprises a receiver configured to receive a firstparameter broadcasted from a current serving cell and a second parameterbroadcasted from the current serving cell, and a controller configuredto reselect a target cell to be used as a serving cell from among aplurality of cells operated at different frequencies. The firstparameter is a parameter that defines a probability for performingreselection of the target cell, and the second parameter is a timervalue to be set in a timer started at a timing at which a qualitymeasurement for neighbor cells ends. The controller is configured to,after receiving the first parameter and the second parameter,periodically measure qualities of neighbor cells at timings defined bythe timer, compare a value corresponding to IMSI (International MobileSubscriber Identity) of the user equipment with a value corresponding tothe first parameter, and reselect the target cell from among cellshaving a quality satisfying predetermined quality criteria, based onresults of the quality measurement and the comparison.

A method according to the present disclosure, which is performed at auser equipment for a mobile communication system, comprises receiving afirst parameter broadcasted from a current serving cell and a secondparameter broadcasted from the current serving cell, and reselecting atarget cell to be used as a serving cell from among a plurality of cellsoperated at different frequencies. The first parameter is a parameterthat defines a probability for performing reselection of the targetcell, and the second parameter is a timer value to be set in a timerstarted at a timing at which a quality measurement for neighbor cellsends. The reselecting comprises, after receiving the first parameter andthe second parameter, periodically measuring qualities of neighbor cellsat timings defined by the timer, comparing a value corresponding to IMSI(International Mobile Subscriber Identity) of the user equipment with avalue corresponding to the first parameter, and reselecting the targetcell from among cells having a quality satisfying predetermined qualitycriteria, based on results of the quality measurement and thecomparison.

A chip according to the present disclosure for a user equipment for amobile communication system comprises at least one processor and atleast one memory. The at least one processor is configured to performprocesses of receiving a first parameter broadcasted from a currentserving cell and a second parameter broadcasted from the current servingcell, and reselecting a target cell to be used as a serving cell fromamong a plurality of cells operated at different frequencies. The firstparameter is a parameter that defines a probability for performingreselection of the target cell, and the second parameter is a timervalue to be set in a timer started at a timing at which a qualitymeasurement for neighbor cells ends. The reselecting comprises, afterreceiving the first parameter and the second parameter, periodicallymeasuring qualities of neighbor cells at timings defined by the timer,comparing a value corresponding to IMSI (International Mobile SubscriberIdentity) of the user equipment with a value corresponding to the firstparameter, and reselecting the target cell from among cells having aquality satisfying predetermined quality criteria, based on results ofthe quality measurement and the comparison.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a configuration diagram of an LTE system according to anembodiment.

FIG. 2 is a block diagram of a UE according to an embodiment.

FIG. 3 is a block diagram of an eNB according to an embodiment.

FIG. 4 is a protocol stack diagram of a radio interface according to anembodiment.

FIG. 5 is a configuration diagram of a radio frame used in an LTE systemaccording to an embodiment.

FIG. 6 is a diagram for describing an application scene according to anembodiment.

FIG. 7 is a sequence diagram illustrating a mobile communication methodaccording to an embodiment.

FIG. 8 is a flowchart illustrating a mobile communication methodaccording to a first modified example.

FIG. 9 is a sequence diagram illustrating a mobile communication methodaccording to a second modified example.

FIGS. 10a to 10c are diagrams according to an additional note of anembodiment.

FIG. 11 is a diagram according to an additional note of an embodiment.

FIGS. 12a to 12c are diagrams according to an additional note of anembodiment.

FIGS. 13a and 13b are diagrams according to an additional note of anembodiment.

FIG. 14 is a diagram according to an additional note of an embodiment.

FIGS. 15a to 15d are diagrams according to an additional note of anembodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a mobile communication system according to an embodimentwill be described with reference to the accompanying drawings. In thefollowing description of the drawings, the same or similar parts aredenoted by the same or similar reference numerals.

It should be noted, however, that the drawings are schematic, and ratiosof dimensions or the like may be different from actual ones. Therefore,specific dimensions or the like should be determined with reference tothe following description. Further, it will be appreciated that parts inwhich relations or ratios of dimensions are different between thedrawings are included.

[Overview of Disclosure]

In the mobile communication system mentioned in the background art, theselection of the target cell to be used as the serving cell is performedon the basis of the quality of the neighbor cell. Therefore, the userterminals located at geographically close positions are likely to selectthe same cell as the target cell. Further, the geographical distributionof the user terminals is often biased. In other words, there is apossibility that the cell to be used as the serving cell by the userterminal may become biased to the same cell, and there is a possibilitythat the load distribution of the respective cells is not properlyperformed.

A user terminal according to an overview of the disclosure includes acontroller that selects a target cell to be used as a serving cell fromamong a plurality of cells operated at different frequencies. Thecontroller measures qualities of neighbor cells adjacent to a currentserving cell at a predetermined timing even when a quality of thecurrent serving cell satisfies predetermined quality criteria.

A user terminal according to an overview of the disclosure includes acontroller that selects a target cell to be used as a serving cell fromamong cells having a quality satisfying predetermined quality criteriaamong a plurality of cells operated at different frequencies. Thecontroller selects the target cell using an identifier allocated to theuser terminal.

A user terminal according to an overview of the disclosure includes acontroller that selects a target cell to be used as a serving cell fromamong a plurality of cells operated at different frequencies. Thecontroller determines whether or not reselection of the target cell isperformed using a value specific to the user terminal.

According to the user terminal of the overview of the disclosure, it ispossible to properly perform the load distribution of the respectivecells. In other words, it is possible to appropriately distribute theuser terminals in the RRC idle state to the respective cells.

[Embodiment]

An LTE system based on the 3GPP standard will be described below as anexample of a mobile communication system.

(System Configuration)

A system configuration of an LTE system according to an embodiment willbe described. FIG. 1 is a configuration diagram of an LTE systemaccording to an embodiment.

The LTE system according to the embodiment includes a user equipment(UE) 100, an evolved-UMTS terrestrial radio access network (E-UTRAN) 10,and an evolved packet core (EPC) 20 as illustrated in FIG. 1.

The UE 100 corresponds to a user terminal. The UE 100 is a mobilecommunication device, and performs radio communication with a cellformed by an evolved Node-B (eNB 200) (a serving cell in a case in whichthe UE 100 is in an RRC connected state). A configuration of the UE 100will be described later.

The E-UTRAN 10 corresponds to a radio access network. The E-UTRAN 10includes the eNB 200. The eNB 200 corresponds to a radio base station.The eNBs 200 are connected to each other via an X2 interface. Aconfiguration of the eNB 200 will be described later.

The eNB 200 forms one or more cells and performs radio communicationwith the UE 100 that has established a connection with a cell thereof.The eNB 200 has a radio resource management (RRM) function, a user datarouting function, a measurement control function for mobilitycontrol/scheduling, and the like. In addition to a term indicating aminimum unit of a radio communication area, “cell” is also used as aterm indicating a function performing radio communication with the UE100.

The EPC 20 corresponds to a core network. The EPC 20 includes a mobilitymanagement entity (MME)/serving-gateway (S-GW) 300. The MME performsvarious kinds of mobility control or the like on the UE 100. The S-GWperforms user data transfer control. The MME/S-GW 300 is connected tothe eNB 200 via an S1 interface. The E-UTRAN 10 and the EPC 20constitute a network of the LTE system.

FIG. 2 is a block diagram of the UE 100. The UE 100 includes a pluralityof antennas 101, a radio transceiver 110, a user interface 120, a globalnavigation satellite system (GNSS) receiver 130, a battery 140, a memory150, and a processor 160 as illustrated in FIG. 2. The memory 150 andthe processor 160 constitute a controller. The radio transceiver 110 andthe processor 160 constitute a transmitter and a receiver. The UE 100may not include the GNSS receiver 130. Further, the memory 150 may beintegrated with the processor 160, and this set (that is, chipset) maybe used as a processor.

The antenna 101 and the radio transceiver 110 are used for transmissionand reception of radio signals. The radio transceiver 110 converts abaseband signal (a transmission signal) output from the processor 160into a radio signal and transmits the radio signal from the antenna 101.In addition, the radio transceiver 110 converts a radio signal receivedby the antenna 101 into a baseband signal (a reception signal) andoutputs the baseband signal to the processor 160.

The user interface 120 is an interface with a user who owns the UE 100,and includes, for example, a display, a microphone, a speaker, variousbuttons, or the like. The user interface 120 receives an operation fromthe user and outputs a signal indicating content of the receivedoperation to the processor 160. The GNSS receiver 130 receives a GNSSsignal and outputs the received GNSS signal to the processor 160 inorder to obtain position information indicating a geographical positionof the UE 100. The battery 140 stores electric power to be supplied tothe blocks of the UE 100.

The memory 150 stores a program executed by the processor 160 andinformation used for a process performed by the processor 160. Theprocessor 160 includes a baseband processor that performsmodulation/demodulation, encoding/decoding, and the like of the basebandsignal and a central processing unit (CPU) that performs various kindsof processes by executing the program stored in the memory 150. Theprocessor 160 may further include a codec that encodes and decodes audioand video signals. The processor 160 executes various kinds of processesto be described later and various kinds of communication protocols.

FIG. 3 is a block diagram of the eNB 200. The eNB 200 includes aplurality of antennas 201, a radio transceiver 210, a network interface220, a memory 230, and a processor 240 as illustrated in FIG. 3. Thememory 230 and the processor 240 constitute a controller. The radiotransceiver 210 (and/or the network interface 220) and the processor 240constitute a transmitter and a receiver. Further, the memory 230 may beintegrated with the processor 240, and this set (that is, chipset) maybe used as a processor.

The antenna 201 and the radio transceiver 210 are used for transmissionand reception of radio signals. The radio transceiver 210 converts abaseband signal (a transmission signal) output from the processor 240into a radio signal and transmits the radio signal from the antenna 201.Further, the radio transceiver 210 converts a radio signal received bythe antenna 201 into a baseband signal (a reception signal) and outputsthe baseband signal to the processor 240.

The network interface 220 is connected to a neighbor eNB 200 via an X2interface and connected to the MME/S-GW 300 via an S1 interface. Thenetwork interface 220 is used for communication performed on the X2interface and communication performed on the S1 interface.

The memory 230 stores a program executed by the processor 240 andinformation used for a process performed by the processor 240. Theprocessor 240 includes a baseband processor that performsmodulation/demodulation, encoding/decoding, and the like of the basebandsignal and a CPU that performs various kinds of processes by executingthe program stored in the memory 230. The processor 240 executes variouskinds of processes to be described later and various kinds ofcommunication protocols.

FIG. 4 is a protocol stack diagram of a radio interface in the LTEsystem. As illustrated in FIG. 4, the radio interface protocol isdivided into first to third layers of an OSI reference model, and thefirst layer is a physical (PHY) layer. The second layer includes amedium access control (MAC) layer, a radio link control (RLC) layer, anda packet data convergence protocol (PDCP) layer. The third layerincludes a radio resource control (RRC) layer.

The physical layer performs encoding/decoding, modulation/demodulation,antenna mapping/demapping, and resource mapping/demapping. User data andcontrol information are transmitted between the physical layer of the UE100 and the physical layer of the eNB 200 via the physical channel.

The MAC layer performs priority control of data, a retransmissionprocess by hybrid ARQ (HARQ), a random access procedure, and the like.User data and control information are transmitted between the MAC layerof the UE 100 and the MAC layer of the eNB 200 via the transportchannel. The MAC layer of the eNB 200 includes a scheduler that decidesuplink and downlink transport formats (a transport block size and amodulation and coding scheme (MCS)) and an allocated resource block forthe UE 100.

The RLC layer transmits data to the RLC layer on a reception side usingthe functions of the MAC layer and the physical layer. User data andcontrol information are transmitted between the RLC layer of the UE 100and the RLC layer of the eNB 200 via a logical channel.

The PDCP layer performs header compression/decompression andencryption/decryption. Further, it should be noted that a transmittingentity of transmitting a data unit (PDCP PDU) or a receiving entity ofreceiving a data unit (PDCP PDU) is formed in the PDCP layer.

The RRC layer is defined only on a control plane for dealing withcontrol information. A control signal (an RRC message) for various kindsof settings is transmitted between the RRC layer of the UE 100 and theRRC layer of the eNB 200. The RRC layer controls logical channels,transport channels, and physical channels in accordance withestablishment, re-establishment, and release of a radio bearer. In acase in which there is a connection (RRC connection) between the RRC ofthe UE 100 and the RRC of the eNB 200, the UE 100 is in an RRC connectedstate, and in a case in which there is no connection (RRC connection)between the RRC of the UE 100 and the RRC of the eNB 200, the UE 100 isin an RRC idle state.

A non-access stratum (NAS) layer located above the RRC layer performssession management, mobility management, and the like.

FIG. 5 is a configuration diagram of a radio frame used in the LTEsystem. In the LTE system, orthogonal frequency division multiple access(OFDMA) is applied to downlink, and Single Carrier Frequency DivisionMultiple Access (SC-FDMA) is applied to uplink.

As illustrated in FIG. 5, the radio frame includes ten subframesarranged in a time direction. Each subframe includes two slots arrangedin the time direction. A length of each subframe is 1 ms, and a lengthof each slot is 0.5 ms. Each subframe includes a plurality of resourceblocks (RB) in a frequency direction and includes a plurality of symbolsin a time direction. Each resource block includes a plurality ofsubcarriers in the frequency direction. One resource element (RE) isconstituted by one symbol and one subcarrier. Among the radio resources(time/frequency resources) allocated to the UE 100, frequency resourcesare able to be specified by resource blocks, and time resources is ableto be specified by subframes (or slots).

(Application Scene)

An application scene will be described below. FIG. 6 is a diagram fordescribing an application scene according to an embodiment.

A plurality of eNBs 200 (for example, an eNB 200#1, an eNB 200#2, an eNB200#3, and an eNB 200#4) are provided as illustrated in FIG. 6. The eNB200#1 includes a cell #1 as a radio communication area, the eNB 200#2includes a cell #2 as the radio communication area, the eNB 200#3includes a cell #3-1 and a cell #3-2 as the radio communication area,and the eNB 200#4 includes a cell #4-1 and a cell #4-2 as the radiocommunication area.

The cell #2, the cell #3-1, the cell #3-2, the cell #4-1, and the cell#4-2 overlap the cell #1. Further, the respective cells are operated atdifferent frequencies. A priority is given to a frequency at which eachcell is operated. A correspondence relation between the frequency andthe priority is included in system information (system information block(SIB)) transmitted from the eNB 200.

Under this assumption, the UE 100 selects a target cell to be used as aserving cell from among a plurality of cells operated at differentfrequencies. Specifically, in a case in which a start condition issatisfied, the UE 100 measures qualities of neighbor cells adjacent to acurrent serving cell and selects a target cell to be used as a servingcell from cells satisfying a selection condition.

Firstly, the start condition is as follows.

(A1) A frequency having a higher priority than a priority of a frequencyof a current serving cell

-   -   The UE 100 constantly measures a quality of the frequency having        the high priority.

(A2) A frequency having a priority equal to or lower than a priority ofa frequency of a current serving cell

-   -   The UE 100 measures a quality of the frequency having the        priority equal to or lower than the priority of the frequency of        the current serving cell in a case in which the quality of the        current serving cell is lower than a predetermined threshold        value.

Secondly, the selection condition is as follows.

(B1) The priority of the frequency of the neighbor cell is higher thanthe priority of the current serving cell

-   -   The UE100 selects a cell satisfying a relation of        Squal>Thresh_(X,HighQ) over a predetermined period        (Treselection_(RAT)) or a cell satisfying a relation of        Srxlev>Thresh_(X,HighP) over a predetermined period        (Treselection_(RAT)) as a target cell. In this case, criteria        that the neighbor cell has to satisfy is also referred to as        “S-criteria.”

Here, Squal indicates a cell selection quality level and is calculatedby Squal=Q_(qualmeas)−(Q_(qualmin)+Q_(qualminoffset))−Qoffset_(temp).Q_(qualmeas) indicates a quality level (RSRQ) of the neighbor cell,Q_(qualmin) indicates a minimum required quality level,Q_(qualmin offset) indicates a predetermined offset which is constantlyapplied to the neighbor cell, and Qoffset_(temp) indicates an offsetwhich is temporarily applied to the neighbor cell. Thresh_(X,HighQ)indicates a predetermined threshold value.

Further, Srxlev indicates a cell selection reception level and iscalculated bySrxlev=Q_(rxlevmeas)−(Q_(rxlevmin)+Q_(rxlevminoffset))−Pcompensation−Qoffset_(temp).Q_(rxlevmeas) indicates a reception level (RSRP) of the neighbor cell,Q_(rxlevmin) indicates a minimum request reception level, Q_(rxlevmin)offset indicates a predetermined offset which is constantly applied tothe neighbor cell, Pcompensation is a parameter related to an uplinkcapability, and Qoffset_(temp) indicates an offset which is temporarilyapplied to the neighbor cell. Thresh_(X,HighP) indicates a predeterminedthreshold value.

(B2) The priority of the frequency of the neighbor cell is equal to thepriority of the current serving cell

-   -   The UE100 calculates a ranking R_(s) of the current serving cell        and a ranking R_(n) of the neighbor cell, and selects a cell        having a higher ranking R_(n) than the ranking R_(s) over a        predetermined period (Treselection_(RAT)) as a target cell. In        this case, criteria that the neighbor cell has to satisfy is        also referred to as “R-criteria.”

Here, R_(s) is calculated by R_(s)=Q_(meas,s)+Q_(Hyst)−Qoffset_(temp).R_(n) is calculated by R_(n)=Q_(meas,n)−Qoffset−Qoffset_(temp).Q_(meas,s) indicates a reception level (RSRP) of the current servingcell, and Q_(meas,n) indicates a reception level (RSRP) of the neighborcell. Q_(Hyst) indicates a hysteresis value for causing the currentserving cell to be more likely to be reselected as the target cell.Qoffset_(temp) indicates an offset which is temporarily applied to thecurrent serving cell and the neighbor cell.

(B3) The priority of the frequency of the neighbor cell is lower thanthe priority of the current serving cell

-   -   The UE100 selects one of neighbor cells as the target cell        through a technique similar to (B1) under the assumption that        Squal<Thresh_(Serving,LowQ) is satisfied over a predetermined        period (Treselection_(RAT)), or Srxlev<Thresh_(Serving,LowP) is        satisfied over a predetermined period (Treselection_(RAT)).

Here, Thresh_(Serving,LowQ) and Thresh_(Serving,LowP) are predeterminedthreshold values, similarly to Thresh_(X,HighQ) and Thresh_(X,HighP).

Further, various kinds of parameters used in the selection of the targetcell are included in system information (system information block (SIB))transmitted from the eNB 200. Various kinds of parameters include apriority of a frequency (cellReselectionPriority), a predeterminedperiod (Treselection_(RAT)), various kinds of offsets(Q_(qualmin offset), Q_(rxlevmin offset), Qoffset_(temp), Q_(Hyst), andQoffset), various kinds of threshold values (Thresh_(X,HighQ),Thresh_(X,HighP), Thresh_(Serving,LowQ), Thresh_(Serving,LowP)).

In the embodiment, the UE 100 (the controller) selects the target cellto be used as the serving cell from a plurality of cells operated atdifferent frequencies. Specifically, even when the quality of thecurrent serving cell satisfies predetermined quality criteria, the UE100(controller) measures the quality of the neighbor cell adjacent to thecurrent serving cell at a predetermined timing. Here, “the predeterminedquality criteria is satisfied” means that the start condition is notsatisfied. In detail, the predetermined quality criteria is a conditionin which the quality of the current serving cell is not lower than apredetermined threshold value as described above in (A2).

In other words, in principle, the UE 100 (the controller) according tothe embodiment is configured not to start the quality measurement forthe neighbor cell adjacent to the current serving cell in a case inwhich the quality of the current serving cell satisfies thepredetermined quality criteria. However, it should be noted that the UE100 (the controller) exceptionally starts the quality measurement forthe neighbor cell on the basis of the system information received fromthe current serving cell as will be described later.

Here, the UE 100 (the controller) specifies a predetermined timing onthe basis of the system information (system information block (SIB))received from the current serving cell. The “predetermined timing”indicates a timing at which the quality measurement for the neighborcell is started as the target cell is selected.

The system information may mean that the quality measurement for theneighbor cell is immediately started. In this case, the UE 100 (thecontroller) specifies a timing at which the system information isreceived as the predetermined timing and starts the quality measurementfor the neighbor cell in response to the reception of the systeminformation.

Alternatively, the system information may include a subframe number fromwhich the quality measurement for the neighbor cell is started. The UE100 (the controller) specifies a subframe number included in the systeminformation as the predetermined timing and starts the qualitymeasurement for the neighbor cell from the specified subframe number.

Alternatively, the system information may include information indicatinga period of the predetermined timing. For example, the systeminformation contains a timer value to be set in a timer started at atiming at which the quality measurement for the neighbor cell ends. TheUE 100 (the controller) specifies a timing at which the timer in whichthe timer value is set expires as the predetermined timing, and startsthe quality measurement for the neighbor cell at the specified timing.

Here, in a case in which the system information includes the informationindicating the period of the predetermined timing, the UE 100 (thecontroller) may correct the period of the predetermined timing on thebasis of a value specific to the UE 100. The value specific to the UE100 may be, for example, a subframe number for receiving the systeminformation or an identifier (UE-ID) of the UE 100. The UE 100 (thecontroller) sets a value obtained by “timer value×UE-ID” in the timerand starts the quality measurement for the neighbor cell at a timing atwhich the timer expires.

Alternatively, the system information may include a threshold value tobe compared with a random number generated by the UE 100 (thecontroller). The UE 100 (the controller) measures the quality of theneighbor cell on the basis of a comparison result between the randomnumber and a threshold value at a predetermined timing. For example, theUE 100 (the controller) starts the quality measurement for the neighborcell in a case in which a condition that the random number is largerthan the threshold value or the random number is smaller than thethreshold value at a predetermined timing is satisfied.

(Mobile Communication Method)

A mobile communication method according to the embodiment will bedescribed below. FIG. 7 is a sequence diagram illustrating a mobilecommunication method according to the embodiment.

As illustrated in FIG. 7, in step S10, the UE 100 is in the RRC idlestate.

In step S11, the UE 100 receives the system information (systeminformation block (SIB)) transmitted from the eNB 200 (current servingcell).

In step S12, the UE 100 measures the quality of the neighbor celladjacent to the current serving cell at a predetermined timing even whenthe quality of the current serving cell satisfies the predeterminedquality criteria. The UE 100 specifies the predetermined timing on thebasis of the system information (system information block (SIB)).

(Operations and Effects)

The UE 100 (the controller) according to the embodiment measures thequality of the neighbor cell adjacent to the current serving cell at apredetermined timing even when the quality of the current serving cellsatisfies the predetermined quality criteria. In other words, even inthe case of the UEs 100 which are located at geographically closepositions, timings at which the quality measurement for the neighborcell is started as the target cell is selected are temporally discrete.Therefore, it is possible to appropriately distribute a load of eachcell. In other words, the UEs 100 in the RRC idle state can beappropriately distributed to the respective cells.

[First Modified Example]

A first modified example of the embodiment will be described below. Thedescription will proceed focusing on differences with the embodiment.

Specifically, in the embodiment, the UE 100 measures the quality of theneighbor cell adjacent to the current serving cell at a predeterminedtiming even when the quality of the current serving cell satisfies thepredetermined quality criteria. On the other hand, in the first modifiedexample, the UE 100 (the controller) selects the target cell to use asthe serving cell from among cells having a quality that satisfies thepredetermined quality criteria (S-criteria or R-criteria) as describedabove in (B1) to (B3) in the embodiment. The UE 100 (the controller)selects the target cell using a randomization value in the selection ofthe target cell.

Preferably, the value for randomizing is not a value common to all theUEs 100 existing in the current serving cell. For example, the value forrandomizing is one or more values selected from an identifier (UE-ID)allocated to the UE 100, a random number generated by the UE 100 (thecontroller), and an access class (AC) related to access restriction ofthe UE 100.

Here, the UE 100 (the controller) may correct the quality (for example,Squal, Srxlev, Q_(meas,s), and Q_(meas,n)) on the basis of therandomization value (for example, the UE-ID, the random number, or theAC). For example, as a quality correction method, new values may bedefined as various kinds of offsets (Q_(qualmin offset),Q_(rxlevmin offset), Qoffset_(temp), Q_(Hyst), and Qoffset), or newoffsets may be introduced. For example, various types of offsets arecalculated by offset=(default offset)×(UE-ID÷n). Here, “n” is apredetermined value or a value transmitted from the serving cell.

Alternatively, the UE 100 (the controller) may correct the priority ofthe frequency (cellReselectionPriority) in which each of a plurality ofcells is operated on the basis of the randomization value (for example,the UE-ID, the random number, or the AC).

Alternatively, the UE 100 (the controller) may select the target cellfrom among cells having the quality satisfying the predetermined qualitycriteria (S-criteria or R-criteria) on the randomization value (forexample, the UE-ID, the random number, or the AC). In other words, theUE 100 (the controller) specifies the cells having the qualitysatisfying the predetermined quality criteria (S-criteria or R-criteria)and selects the target cell among the specified cells. For example, theUE 100 (the controller) corrects the ranking of the selection candidatecell on the basis of the randomization value.

Alternatively, the UE 100 (the controller) may select the target cellfrom among cells operated at the frequency having the same priority asthe frequency of the current serving cell on the basis of therandomization value. Here, the frequency having the same priority as thefrequency of the current serving cell may be the same frequency as thefrequency of the current serving cell or may be different from thefrequency of the current serving cell. In other words, the UE 100 (thecontroller) specifies the cells satisfying the above-described conditionand selects the target cell on the basis of the randomization value fromamong the specified cells (selection candidate cells). For example, theUE 100 (the controller) corrects the ranking of the selection candidatecell on the basis of the randomization value.

Alternatively, the UE 100 (the controller) may select the target cell onthe basis of the randomization value from among cells having a qualityof a predetermined range. Here, the predetermined range is preferablyincluded in the system information (system information block (SIB))transmitted from the current serving cell. In other words, the UE 100(the controller) may specify the cell having the quality of thepredetermined range, select the target cell from among the specifiedcells (selection candidate cells) on the basis of the randomizationvalue. The quality of the predetermined range may be a quality whosedifference with the highest quality falls within the predetermined range(for example, within 5 dB) or may be a quality of a cell having aranking of a predetermined range (for example, top three) in thedescending order of rankings. For example, the UE 100 (the controller)corrects the ranking of the selection candidate cell on the basis of therandomization value.

For example, the ranking of the cell may be corrected as follows. In acase in which the random number is used as the randomization value, theranking of the cell is changed using a function of Roundup {RAND×(thenumber of selection candidate cells)}. Alternatively, in a case in whichthe UE-ID is used as the randomization value, the cell which has thehighest ranking among the selection candidate cells and satisfies arelation of (UE ID)mod(Cell ID)<n is selected as the target cell. Here,“n” is a predetermined value or a value transmitted from the servingcell.

(Mobile Communication Method)

A mobile communication method according to the first modified examplewill be described below. FIG. 8 is a flowchart illustrating the mobilecommunication method according to the first modified example. The flowillustrated in FIG. 8 is performed by the UE 100.

As illustrated in FIG. 8, in step S20, the UE 100 determines whether ornot the start condition for starting the measurement of the quality ofthe neighbor cell adjacent to the current serving cell is satisfied. Ina case in which a determination result is YES, the UE 100 performs aprocess of step S21. On the other hand, in a case in which thedetermination result is NO, the UE 100 ends a series of processes.

As described above, the start condition is as follows.

(A1) A frequency having a higher priority than a priority of a frequencyof a current serving cell

-   -   The UE 100 constantly measures a quality of the frequency having        the high priority.

(A2) A frequency having a priority equal to or lower than a priority ofa frequency of a current serving cell

-   -   The UE 100 measures a quality of the frequency having the        priority equal to or lower than the priority of the frequency of        the current serving cell in a case in which the quality of the        current serving cell is lower than a predetermined threshold        value.

Here, in step S20, as described above, the priority of the frequency(cellReselectionPriority) may be corrected on the basis of therandomization value (for example, the UE-ID, the random number, or theAC).

In step S21, the UE 100 measures the quality of the neighbor celladjacent to the current serving cell.

Here, in step S21, as described above, the quality (for example, Squal,Srxlev, Q_(meas,s), and Q_(meas,n)) may be corrected on the basis of therandomization value (for example, the UE-ID, the random number, or theAC).

In step S22, the UE 100 selects the target cell to be used as theserving cell from among the cells (selection candidate cells) having thequality satisfying the predetermined quality criteria (S-criteria andR-criteria).

Here, in step S22, as described above, the ranking of the selectioncandidate cell may be corrected on the basis of the randomization value(for example, the UE-ID, the random number, or the AC).

In the description of FIG. 8, only a part of the first modified exampleis illustrated, but it should be noted that it is preferable for the UE100 to select the target cell using the randomization value as describedabove.

(Operations and Effects)

The UE 100 (the controller) according to the first modified exampleselects the target cell using the randomization value. In other words,even in the case of the UEs 100 which are located at geographicallyclose positions, the cell selected as the target cell varies. Therefore,it is possible to appropriately distribute the load of each cell. Inother words, it is possible to appropriately distribute the UEs 100 inthe RRC idle state to the respective cells.

[Second Modified Example]

A second modified example of the embodiment will be described below. Thedescription will proceed with differences with the embodiment.

Specifically, in the embodiment, the UE 100 measures the quality of theneighbor cell adjacent to the current serving cell at a predeterminedtiming even when the quality of the current serving cell satisfies thepredetermined quality criteria. On the other hand, in the secondmodified example, as described above (B1) to (B3) in the embodiment, theUE 100 (the controller) selects the target cell to be used as theserving cell from among the cells having the quality satisfying thepredetermined quality criteria (S-criteria or R-criteria). The UE 100(the controller) selects the target cell on the basis of a differentreselection parameter for each group including one or more UEs 100.

Here, a group to which the UE 100 belongs is designated through amessage (for example, an RRC Connection Release) used in a transitionprocedure from the connected state to the idle state. The message (forexample, the RRC Connection Release) may include group identificationinformation identifying the group to which the UE 100 belongs.

Further, the reselection parameter includes a parameter of a frequency(cellReselectionPriority), a predetermined period (Treselection_(RAT)),various kinds of offsets (Q_(qualmin offset), Q_(rxlevmin offset),Qoffset_(temp), Q_(Hyst), and Qoffset), and various kinds of thresholdvalues (Thresh_(X,HighQ), Thresh_(X,HighP), Thresh_(Serving,LowQ), andThresh_(Serving,LowP)).

In the second modified example, the reselection parameter is preferablyincluded in the system information (system information block (SIB))transmitted from the current serving cell. In other words, a cell thattransmits the reselection parameter may be different from a cell thattransmits a message (for example, the RRC Connection Release). Here, thereselection parameter may be included in a message (for example, the RRCConnection Release).

Here, the UE 100 (the controller) may maintain the group to which the UE100 belongs until transition from the RRC idle state to the RRCconnected state is performed. In other words, the group to which the UE100 belongs is released by the transition from the RRC idle state to theRRC connected state. Alternatively, the UE 100 (the controller) maymaintain the group to which the UE 100 belongs until a timer activatedby receipt of the message (for example, the RRC Connection Release)expires. In other words, the group to which the UE 100 belongs may bereleased in a case in which a timer started by designation of a groupexpires.

The group including one or more UEs 100 may be formed by one or morepieces of information selected from a category of the UE 100 (themagnitude of a throughput), a capability of the UE 100 (the number ofstreams of MIMO or the like), statistics related to traffic of the UE100 (S1 Initial UE Context Setup), statistics related to mobility of theUE 100 (S1 Initial UE Context Setup), and position information of the UE100 (including the measurement report).

Alternatively, the group including one or more UEs 100 may be formed bythe access class (AC) related to the access restriction of the UE 100.In other words, the reselection parameter differs according to eachaccess class related to the access restriction of the UE 100.

(Mobile Communication Method)

A mobile communication method according to the second modified examplewill be described below. FIG. 9 is a sequence diagram illustrating themobile communication method according to the second modified example.

As illustrated in FIG. 9, in step S30, the UE 100 is in the RRCconnected state.

In step S31, the UE 100 receives the message (RRC Connection Release)used in the transition procedure from the connected state to the idlestate. The message (RRC Connection Release) designates the group towhich the UE 100 belongs.

In step S32, the UE 100 is in the RRC idle state.

In step S33, the UE 100 receives the system information (systeminformation block (SIB)) transmitted from the eNB 200 (the currentserving cell). The system information includes a different reselectionparameter for each group including one or more UEs 100.

A cell that transmits the reselection parameter may be different from acell that transmits the message (for example, the RRC ConnectionRelease).

In step S34, the UE 100 selects the target cell on the basis of thereselection parameter allocated to the group to which the UE 100belongs.

(Operations and Effects)

The UE 100 (the controller) according to the second modified exampleselects the target cell on the basis of the different reselectionparameter for each group including one or more UEs 100. In other words,even in the case of the UEs 100 which are located at geographicallyclose positions, the cell selected as the target cell varies for eachgroup. Therefore, it is possible to appropriately distribute the load ofeach cell. In other words, it is possible to appropriately distributethe UEs 100 in the RRC idle state to the respective cells.

[Third Modified Example]

A third modified example of the embodiment will be described below. Thedescription will proceed with differences with the first modifiedexample.

In the first modified example, the case in which the UE 100 (thecontroller) corrects the ranking of the selection candidate cell on thebasis of the randomization value has been described. On the other hand,in the third modified example, another selection method will bedescribed as the target cell selection method based on the randomizationvalue.

Specifically, the UE 100 (receiver) receives the system informationincluding a threshold value to be compared with a random numbergenerated by the controller. The UE 100 (the controller) selects thetarget cell on the basis of a comparison result between the randomnumber and the threshold value.

More specifically, as described above, the UE 100 (the controller)calculates the ranking of each of a plurality of cells on the basis of ameasurement result of the quality of each of a plurality of cells. Therankings are the ranking R_(s) of the current serving cell and theranking R_(n) of the neighbor cell as described above. The UE 100 (thecontroller) selects the target cell on the basis of the comparisonresult between the random number and the threshold value.

In this case, the system information includes a different thresholdvalue for each ranking. The UE 100 (the controller) generates the randomnumber for each ranking and selects the target cell on the basis of thecomparison result between the random number of each ranking and thethreshold value. The UE 100 (the controller) may compare the randomnumber with the threshold value in the descending order of rankings andselect a cell in which the comparison result between the random numberand the threshold value satisfies the selection condition as the targetcell.

For example, the system information includes information (Ranking_No,Probability) in which the ranking is associated with the thresholdvalue. The information (Ranking_No, Probability) is information such as{Rank 1, 0.6}, {Rank 2, 0.8}, and {Rank 3, 1.0}. {Rank 1, 0.6} indicatesthat the threshold value associated with Rank 1 is 0.6. Similarly, {Rank2, 0.8} indicates that the threshold value associated with Rank 2 is0.8, and {Rank 3, 1.0} indicates that the threshold value associatedwith Rank 3 is 1.0. Further, a range of the random number generated bythe UE 100 (the controller) is 0 to 1. Here, the selection conditionthat the comparison result between the random number and the thresholdvalue has to satisfy is, for example, a condition that the random numberis the threshold value or less.

Firstly, the UE 100 (the controller) determines whether or not a cell ofranking 1 is selected as the target cell. In other words, the UE 100(the controller) generates a random number for the cell of ranking 1 anddetermines whether or not the comparison result between the randomnumber and 0.6 satisfies the selection condition. For example, in a casein which the random number is 0.8, the UE 100 (the controller)determines that the selection condition is not satisfied. Therefore, theUE 100 (the controller) continues the selection of the target cell.

Secondly, the UE 100 (the controller) determines whether or not a cellof ranking 2 is selected as the target cell. In other words, the UE 100(the controller) generates a random number for the cell of ranking 2 anddetermines whether or not the comparison result between the randomnumber and 0.8 satisfies the selection condition. For example, in a casein which the random number is 0.7, the UE 100 (the controller)determines that the selection condition is satisfied. Therefore, the UE100 (the controller) selects the cell of ranking 2 as the target celland ends the selection of the target cell.

According to the target cell selection method, a probability that thecell of ranking 1 will be selected as the target cell is 60%. Theprobability that the ranking 2 cell will be selected as the target cellis 32% ({1−0.6}×0.8). The probability that the ranking 3 cell will beselected as the target cell is 8% (1−0.6−0.32). As described above, thethreshold value included in the system information may be decided sothat a cell having a high ranking is more likely to be selected as thetarget cell. However, the third modified example is not limited to thisexample, and the threshold value included in the system information maybe decided depending on a load or a capability of a cell.

In the third modified example, the case in which the system informationincludes the information (Ranking_No, Probability) in which the rankingis associated with the threshold value has been described. However, thethird modified example is not limited to this example. For example, thesystem information includes information (Priority, Probability) in whicha priority is associated with a threshold value.

In this case, the UE 100 (the controller) generates a random number foreach priority and selects the target cell on the basis of a comparisonresult between the random number and the threshold value for eachpriority. The UE 100 (the controller) may compare the random number withthe threshold value in the descending order of priorities and select thecell in which the comparison result between the random number and thethreshold value satisfies the selection condition as the target cell.

The priority may be the priority of the frequency at which the cell isoperated or may be a priority specific to the cell.

[Fourth Modified Example]

The fourth modified example of the embodiment will be described below.The description will proceed with difference with the embodiment and thethird modified example.

In the fourth modified example, the UE 100 (the controller) triggers(performs or starts, the same hereinafter) an operation (or a procedure,the same hereinafter) of selecting (or reselecting, the samehereinafter) the target cell in accordance with a reference triggernotification which is one of a plurality of trigger notificationstransmitted from the current serving cell. Preferably, the referencetrigger notification is any one of a plurality of trigger notifications.Here, the reference trigger notification may be some triggernotifications among a plurality of trigger notifications or may be twoor more trigger notifications.

Here, the trigger notification may be a notification which is used fordirectly or indirectly triggering an operation in which the UE 100selects the target cell in a network (here, current servingcell)-initiated manner and transmitted twice or more within a certainperiod of time. In other words, the trigger notification may be anotification for instructing the UE 100 to perform an operation ofselecting the target cell.

For example, the trigger notification may be the system information(system information block (SIB)) described in the embodiment. The systeminformation described in the embodiment includes information specifyinga predetermined timing at which the UE 100 starts quality measurementfor the neighbor cell. Therefore, it should be noted that the systeminformation described in the embodiment directly triggers the operationin which the UE 100 selects the target cell.

Alternatively, the trigger notification may be the system information(system information block (SIB)) described in the third modifiedexample. As described above, the system information described in thethird modified example includes the threshold value to be compared withthe random number. Alternatively, the system information described inthe third modified example may include the information (Ranking_No,Probability) in which the ranking is associated with the thresholdvalue. Alternatively, the system information described in the thirdmodified example may include the information (Priority, Probability) inwhich the priority is associated with the threshold value. In responseto the reception of the system information described in the thirdmodified example, the UE 100 (the controller) selects the cell in whichthe comparison result between the random number and the threshold valuesatisfies the selection condition as the target cell. Therefore, itshould be noted that the system information described in the thirdmodified example indirectly triggers the operation in which the UE 100selects the target cell.

In this case, each of a plurality of trigger notifications includes acounter value. The UE 100 (the controller) stores the counter valueincluded in the reference trigger notification in a counter inaccordance with a trigger (or execution, the same hereinafter) of theoperation of selecting the target cell. For example, the UE 100 retainsthe counter in the memory 150. The UE 100 may accumulate the countervalue in the counter each time the UE 100 triggers the operation ofselecting the target cell in accordance with the reference triggernotification. In a case in which the counter value included in thetrigger notification transmitted from the current serving cell coincideswith the counter value stored in the counter, the UE 100 (thecontroller) does not trigger (prohibits the execution of) the operationof selecting the target cell in accordance with the triggernotification. On the other hand, in a case in which the counter valueincluded in the trigger notification transmitted from the currentserving cell does not coincide with the counter value stored in thecounter, the UE 100 (the controller) triggers the operation of selectingthe target cell. Accordingly, the operation in which the UE 100 selectsthe target cell is triggered only once in accordance with any onetrigger notifications (that is, the reference trigger notification)among the trigger notifications which are transmitted twice or morewithin a certain period of time. In other words, a situation in whichthe operation of selecting the target cell is triggered twice or more inaccordance with each of a plurality of trigger notifications within acertain period is suppressed. In other words, the UE 100 triggers theoperation of selecting the target cell at most once in accordance withone type of trigger notification. The counter value may be apredetermined numerical value (0 to 9 or the like).

Further, the counter value included in each of a plurality of triggernotifications is updated by the current serving cell at the timing atwhich the UE 100 triggers the operation of selecting the target cell. Inother words, the current serving cell transmits the trigger notificationincluding the updated counter value twice or more within a certainperiod of time at a timing at which the UEs 100 in the RRC idle stateare desired to be redistributed to the respective cells.

(Operations and Effects)

Here, if the UE 100 triggers the operation of selecting the target celltwice or more in accordance with each of a plurality of triggernotifications, most of the UEs 100 existing in the current serving cellselect the target cell. In other words, most of the UEs 100 select asimilar cell as the serving cells, and the UEs 100 in the RRC idle stateare not able to be appropriately allocated to the respective cellsaccordingly.

On the other hand, in the fourth modified example, the UE 100 (thecontroller) triggers the operation of selecting the target cell inaccordance with the reference trigger notification which is one of aplurality of trigger notifications transmitted from the current servingcell. Therefore, it is possible to appropriately distribute the UEs 100in the RRC idle state to the respective cells.

[Fifth Modified Example]

A fifth modified example of the embodiment will be described below. Thedescription will proceed with differences with the fourth modifiedexample.

In the fourth modified example, the trigger notification is the systeminformation (system information block (SIB)) described in the embodimentor the third modified example. On the other hand, in the fifth modifiedexample, the trigger notification is a reselection request signal forrequesting reselection of the target cell.

In the fifth modified example, the UE 100 (the receiver) receives thereselection request signal for requesting the reselection of the targetcell from the current serving cell. In response to the reception of thereselection request signal, the UE 100 (the controller) performs thereselection of the target cell. Here, it should be noted that the UE 100(the controller) starts the quality measurement for the neighbor cellassociated with the reselection of the target cell even through thestart condition described in (A1) and (A2) is not satisfied.

Here, the current serving cell transmits the reselection request signalin a case in which the load of the current serving cell is apredetermined load or higher. It is preferable for the current servingcell to repetitively transmit the reselection request signal in a periodof time in which the load of the current serving cell is a predeterminedload or higher. In other words, the UE 100 (the receiver) receives thereselection request signal from the current serving cell in a case inwhich the load of the current serving cell is a predetermined load orhigher. It is preferable for the UE 100 (the receiver) to repeatedlyreceive the reselection request signal in a period of time in which theload of the current serving cell is a predetermined load or higher.

In the fifth modified example, the reselection request signal includes areselection parameter for causing the current serving cell to be lesslikely to be selected as the target cell. The UE 100 (the controller)selects the target cell on the basis of the reselection parameter.Specifically, the reselection parameter may be a parameter of performingdesignation so that the priority of the frequency of the current servingcell (cellReselectionPriority) is changed to the lowest priority or maybe an offset indicating a step number for decreasing the priority of thefrequency (cellReselectionPriority) of the current serving cell.Alternatively, the reselection parameter may include various kinds ofoffsets (Q_(qualmin offset), Q_(rxlevmin offset), Qoffset_(temp),Q_(Hyst), and Qoffset) and various kinds of threshold values(Thresh_(X,HighQ), Thresh_(X,HighP), Thresh_(Serving,LowQ), andThresh_(Serving,LowP)).

In the fifth modified example, the reselection parameter applied inresponse to the reception of the reselection request signal may not beincluded in the reselection request signal but may be included in thesystem information (system information block (SIB)) transmitted from thecurrent serving cell separately from the reselection request signal.Alternatively, the reselection parameter applied in response to thereception of the reselection request signal may be decided in advance.In this case, the UE 100 (the controller) performs the reselection ofthe target cell on the basis of the reselection parameter transmittedfrom the current serving cell or the predetermined reselection parameterin response to the reception of the reselection request signal.

In the fifth modified example, in a case in which the predeterminedcondition is satisfied even through the UE 100 (the controller) receivesthe reselection request signal, the UE 100 (the controller) may keepstaying in the current serving cell without performing the reselectionof the target cell. The “predetermined condition” is a condition relatedto at least one piece of information of a type of UE 100 (UE class), thepriority of the frequency of the current serving cell, a powerconsumption setting of the UE 100, a period of time elapsed after theprevious reselection request signal is received, and a value havingrandomness.

For example, in a case in which the type (UE class) of UE 100 is machinetype communication (MTC) or a dedicated data terminal, since the amountof communication data is small, it is preferable for the UE 100 (thecontroller) to keep existing in the serving cell without performing thereselection of the target cell.

Alternatively, in a case in which the priority of the frequency of thecurrent serving cell is the highest priority, there is a highpossibility that MBMS data is being received or a D2D neighbor service(D2D ProSe) is being provided, it is preferable for the UE 100 (thecontroller) to keep existing in the current serving cell withoutperforming the reselection of the target cell.

Alternatively, in a case in which the power consumption setting of theUE 100 is a low power consumption setting, in order to suppress thepower consumption, it is preferable for the UE 100 (the controller) tokeep existing in the current serving cell without performing thereselection of the target cell.

Alternatively, in a case in which a period of time elapsed since theprevious reselection request signal is received is less than apredetermined period of time, in order to suppress a ping-pongphenomenon, it is preferable for the UE 100 (the controller) to keepexisting in the current serving cell without performing the reselectionof the target cell. For example, it is preferable for the UE 100 (thecontroller) to start a timer at a timing at which the previousreselection request signal is received and not to perform thereselection of the target cell until the timer expires.

Alternatively, in order to suppresses a situation in which a pluralityof UEs 100 perform the reselection of target cells at the same time andrandomly distribute the UEs 100 to the respective cells, in a case inwhich the value having the randomness is not a predetermined value, itis preferable for the UE 100 (the controller) to keep existing in thecurrent serving cell without performing the reselection of the targetcell. Preferably, the value having the randomness is not a value commonto all the UEs 100 existing in the current serving cell. For example,the value having the randomness may be a subframe number (SFN) in whichthe UE 100 receives the reselection request signal, an identifier(UE-ID) of the UE 100 that receives the reselection request signal, or arandom number generated by the UE 100. For example, the UE 100 (thecontroller) performs the reselection of the target cell in a case inwhich SFNmodUE-ID (÷n)=0 is satisfied, and the UE 100 (the controller)does not perform the reselection of the target cell in a case in whichSFNmodUE-ID (÷n)=0 is not satisfied. Here, “n” is a predetermined valueor a value transmitted from the serving cell.

[Sixth Modified Example]

A fifth modified example of the embodiment will be described below. Thedescription will proceed with difference with the embodiment and thefirst to fifth modified examples.

Specifically, in the embodiment and, the first to fifth modifiedexamples, a signal such as the system information (system informationblock (SIB)) is transmitted from the current serving cell to the UE 100.

On the other hand, in the sixth modified example, the signal such as thesystem information transmitted from the current serving cell istransmitted to the UE 100 and transmitted to the neighbor cell (eNB 200)adjacent to the current serving cell (eNB 200) as well. Alternatively,the load state of the current serving cell (eNB 200) is also transmittedto the neighbor cell (eNB 200) adjacent to the current serving cell (eNB200). The signal such as the system information and the load state areexchanged between cells via the X2 interface connecting two or more eNBs200. Accordingly, since the signal such as the system informationtransmitted in each cell (a parameter for encouraging the reselection oftarget cell) and the load states of the respective cells are shared bythe respective cells, it is possible to perform cooperative control of aplurality of cells.

For example, in a case in which all loads of a plurality of cells whichare adjacent to one another are high, it is possible to suppress aphenomenon (ping-pong phenomenon) that the reselection of the targetcell is frequently performed among a plurality of cells. In detail, in acase in which the signal such as the system information (the parameterfor encouraging the reselection of target cell) is transmitted in theneighbor cell or in a case in which the load of the neighbor cell ishigher than a threshold value, the current serving cell holds thetransmission of the signal such as the system information (the parameterfor encouraging the reselection of target cell) to the UE 100.

Here, the signal such as the system information may include theparameter specifying a timing (specific timing) at which the qualitymeasurement for the neighbor cell is started as described above in theembodiment. Alternatively, the signal such as the system information mayinclude the predetermined range which is referred to when the targetcell is selected on the basis of the value having the randomness asdescribed above in the first modified example. Alternatively, the signalsuch as the system information may include the different reselectionparameter for each group including one or more UEs 100 as describedabove in the second modified example. Alternatively, the signal such asthe system information may include the threshold value to be comparedwith the random number generated by the controller as described above inthe third modified example. Alternatively, the signal such as the systeminformation may include the trigger notification for encouraging thereselection of the target cell as described above in the fourth modifiedexample. Alternatively, the signal such as the system information mayinclude the reselection request signal for encouraging the reselectionof the target cell or the reselection parameter applied in response tothe reception of the reselection request signal as described above inthe fifth modified example.

[Seventh Modified Example]

A seventh modified example of the embodiment will be described below.The description will proceed with difference with the embodiment.

In the embodiment, the randomization value includes the random numbergenerated by the UE 100 (the controller). On the other hand, in theseventh modified example, the value having the randomness does notinclude the random number generated by the UE 100 (the controller).Further, the UE 100 determines whether or not the reselection of thetarget cell is performed using the value having the randomness.

For example, the value having the randomness is a value specific to theUE 100 (UE-ID). The value (UE-ID) specific to the UE 100 may be anidentifier allocated by the current serving cell (radio networktemporary identifier (RNTI)), an identifier uniquely allocated to the UE100 in advance (international mobile subscriber identity (IMSI)), or anidentifier allocated in accordance with the location registration of theUE 100 (SAE-temporary mobile subscriber identity (S-TMSI)).

In the seventh modified example, the UE 100 determines whether or notthe reselection of the target cell is performed on the basis of thevalue specific to the UE 100 and the value received from the currentserving cell. Specifically, the value received from the current servingcell includes a value (Np) defining a probability of the reselection ofthe target cell and a value (Nr) for securing fairness between the UEs100. “Np” and “Nr” may be included in announcement informationtransmitted from the current serving cell. In order to ensure thefairness between the UEs 100, it is preferable to change Nr with apredetermined cycle.

For example, the UE 100 may perform the reselection of the target cellin a case in which a condition of (UE-IDmodNp)=Nr is satisfied. In thiscase, the UE 100 does not perform the reselection of the target cell ina case in which the condition of (UE-IDmodNp)=Nr is not satisfied. Asdescribed above, it is possible to use the RNTI, the IMSI, or the S-TMSIas the UE-ID. Therefore, in the above-mentioned equation, the UE-ID maybe replaced with any one of the RNTI, the IMSI, and the S-TMSI.

Here, a timing at which it is determined whether or not the reselectionof the target cell is performed using the value having the randomnessmay be the “predetermined timing” described above in the embodiment. Forexample, the predetermined timing is specified on the basis of thesystem information (system information block (SIB)) received from thecurrent serving cell. Further, the “predetermined timing” indicates atiming at which the quality measurement for the neighbor cell is startedas the target cell is selected.

As described above in the embodiment, the system information mayindicate that the quality measurement for the neighbor cell isimmediately started. In this case, the UE 100 specifies a timing atwhich the system information is received as the predetermined timing andstarts the quality measurement for the neighbor cell in response withthe reception of the system information.

Alternatively, the system information may include a subframe number fromwhich the quality measurement for the neighbor cell is started. The UE100 specifies a subframe number included in the system information asthe predetermined timing and starts the quality measurement for theneighbor cell from the specified subframe number.

Alternatively, the system information may include information indicatinga period of the predetermined timing. For example, the systeminformation contains a timer value to be set in a timer started at atiming at which the quality measurement for the neighbor cell ends. TheUE 100 specifies a timing at which the timer in which the timer value isset expires as the predetermined timing, and starts the qualitymeasurement for the neighbor cell at the specified timing.

In the seventh modified example, the case in which the value (Nr) forsecuring the fairness between the UEs 100 is included in theannouncement information transmitted from the current serving cell hasbeen described. However, the seventh modified example is not limited tothereto. For example, a frame number or a subframe number including apredetermined timing may be used as “Nr.” For example, in a case inwhich the system information indicates that the quality measurement forthe neighbor cell is immediately started, a frame number or a subframenumber in which the system information is received is used as “Nr.”Alternatively, in a case in which the system information includes asubframe number from which the quality measurement for the neighbor cellis started, a frame number or a subframe number from the qualitymeasurement for the neighbor cell is started is used as “Nr.”

Alternatively, a value (systemInfoValueTag) incremented with the updateof the contents of the system information (SIB) may be used as the value(Nr) for securing the fairness between the UEs 100. systemInfoValueTagis included in the announcement information (SIB 1) transmitted from thecurrent serving cell.

It is preferable to select the value of “Nr” so that the relation ofNr<Np is satisfied. Alternatively, the UE 100 may perform thereselection of the target cell in a case in which the condition of(UE-IDmodNp)=(NrmodNp) is satisfied. In this case, the UE 100 does notperform the reselection of the target cell in a case in which thecondition of (UE-IDmodNp)=(NrmodNp) is not satisfied.

Here, the UE 100 may perform the reselection of the target cell in acase in which one or more conditions selected from three types ofconditions (UE-IDmodNp)>(NrmodNp), (UE-IDmodNp)<(NrmodNp), and(UE-IDmodNp)=(NrmodNp) are satisfied. One or more conditions (that is,an equality sign or an inequality sign in the above equations) selectedfrom the three types of conditions may be included in the announcementinformation transmitted from the current serving cell.

Alternatively, a plurality of values Nr (for example, Nr1 and Nr2) maybe set as the value (Nr) for securing the fairness between the UEs 100.In this case, the UE 100 may perform the reselection of the target cellin a case in which a condition of (UE-IDmodNp)=(Nr1 modNp) or acondition of (UE-IDmodNp)=(Nr2 modNp) is satisfied.

Alternatively, a plurality of values Np (for example, Np1 and Np2) maybe set as the value (Np) defining the probability of the reselection ofthe target cell. In this case, the UE 100 may perform the reselection ofthe target cell in a case in which a condition of(UE-IDmodNp1)=(NrmodNp1) or a condition of (UE-IDmodNp2)=(NrmodNp2) issatisfied.

[Other Embodiments]

The present disclosure has been described in accordance with theabove-described embodiments, but it should not be understood that thedescription and the drawings constituting a part of this disclosurelimit the present disclosure. Various alternative embodiments, examples,and operational techniques will be apparent to those skilled in the artfrom this disclosure.

Although not specifically mentioned in the embodiment, the qualitymeasurement for the neighbor cell associated with the selection of thetarget cell may be started in a case in which a predetermined conditionis satisfied at a predetermined timing (the embodiment). Alternatively,the selection of the target cell using the randomization value (thefirst modified example) may be performed when a predetermined conditionis satisfied. For example, the predetermined condition may be acondition that the access class of the UE 100 is an access classtransmitted from the current serving cell. Alternatively, thepredetermined condition may be a condition that priority modAC=0 of thefrequency is satisfied or a condition that cell-IDmodAC=0 is satisfied.

As described above in the embodiment, the entire target cell selectionprocedure includes (A) the procedure (measurement procedure) ofmeasuring the quality of the neighbor cell adjacent to the currentserving cell in a case in which the start condition is satisfied and (B)the procedure (selection procedure) of selecting the target cell to beused as the serving cell from among the cells satisfying the selectioncondition. The embodiment relates to the measurement procedure, and thefirst to third modified examples relates to the selection procedure. Inthis case, the measurement procedure according to the embodiment may becombined with the selection procedure according to any one of the firstto third modified examples. Further, the selection procedures accordingto two or more modified examples selected from the first to thirdmodified examples may be combined.

In the fourth modified example, the case in which the current servingcell causes the UE 100 to trigger the operation of selecting the targetcell in accordance with the reference trigger notification which is oneof a plurality of trigger notifications using the counter valuesincluded in the counter of the UE 100 and the trigger notification hasbeen described. However, the fourth modified example is not limitedthereto. Specifically, it is preferable for the UE 100 to trigger theoperation of selecting the target cell in response to one triggernotification (reference trigger notification) among triggernotifications transmitted from the current serving cell twice or morewithin a certain period of time. Therefore, the UE 100 may be configuredto activate the timer in accordance with the trigger (or the executionor the start, the same hereinafter) of the operation of selecting thetarget cell and not to trigger the operation of selecting the targetcell even though the trigger notification is received before the timerexpires. Preferably, a period of time before the timer expires after thetimer is activated is the same to a period of time (the above-describedcertain period of time) in which the trigger notification is transmittedtwice or more. Alternatively, each of a plurality of triggernotifications may include toggle information capable of having two ormore values, and the UE 100 (the controller) may store toggleinformation included in a reference trigger in a storage area in a casein which the operation of selecting the target cell is triggered inaccordance with the reference trigger notification. In this case, the UE100 (the controller) does not trigger the operation of selecting thetarget cell in a case in which the toggle information included in thetrigger notification transmitted from the current serving cell coincideswith the toggle information stored in the storage area. On the otherhand, the UE 100 (the controller) triggers the operation of selectingthe target cell in a case in which the toggle information included inthe trigger notification transmitted from the current serving cell doesnot coincide with the toggle information stored in the storage area.

Although not particularly mentioned in the embodiment, a cell(hereinafter, an overload cell) that transmits the trigger notificationsuch as the parameter for encouraging the reselection of the target cellor the reselection request signal for requesting the reselection of thetarget cell may be dealt with as follows. Specifically, the UE 100 mayexclude the overload cell from the measurement target cell.Alternatively, the UE 100 may exclude the overload cell from the targetcell to be used as the serving cell even though the overload cellsatisfies the criteria (“S-criteria”) that the neighbor cell has tosatisfy. Alternatively, the UE 100 may exclude the overloaded cell fromthe cell having the ranking. Accordingly, the ping-pong phenomenon thatthe UEs 100 alternately select the neighbor cells as the target cell ina state in which a plurality of neighbor cells are transmitting thetrigger notification (for example, a state in which all of a pluralityof neighbor cells are high in load) is suppressed.

Although not particularly mentioned in the embodiment, the UE 100 mayperform the reselection of the target cell in a case in which thecondition that the period of time elapsed since the UE 100 enters thecurrent serving cell exceeds a time threshold value (for example, 1second) is satisfied. Under this assumption, in a case in which the UE100 performs the reselection of the target cell in accordance with thetrigger notification, it is preferable to use a time threshold valuelarger than a time threshold value used in the case of performing thereselection of the target cell in accordance with the above-describedstart condition (A1 or A2). Accordingly, the ping-pong phenomenon thatthe UEs 100 alternately select the neighbor cells as the target cell ina state in which a plurality of neighbor cells are transmitting thetrigger notification (for example, a state in which all of a pluralityof neighbor cells are high in load) is suppressed. From the same pointof view, in a case in which the UE 100 performs the reselection of thetarget cell in accordance with the trigger notification, it ispreferable to use a predetermined period (Treselection_(RAT)) largerthan a predetermined period (Treselection_(RAT)) used in the case ofperforming the reselection of the target cell in accordance with theabove-described start condition (A1 or A2).

Although not specifically mentioned in the embodiment, a program causinga computer to execute the processes performed by the UE 100 and the eNB200 may be provided. Further, the program may be recorded on a computerreadable medium. It is possible to install the program in the computerusing the computer readable medium. Here, the computer readable mediumon which the program is recorded may be a non-transitory recordingmedium. The non-transitory recording medium is not particularly limitedand may be, for example, a recording medium such as a CD-ROM or aDVD-ROM.

Alternatively, a chip configured with a memory that stores a program forexecuting the processes performed by the UE 100 and the eNB 200 and aprocessor that executes the program stored in the memory may beprovided.

In the embodiment, the LTE system has been described as an example ofthe mobile communication system. However, the embodiment is not limitedthereto. The mobile communication system may be systems other than theLTE system.

[Additional Note 1]

(1. Introduction)

The new work item on Multicarrier Load Distribution of UEs in LTE isapproved and the objective of this work item is to enhance the cellreselection mechanism in IDLE under up-to-date multicarrier operations.

The WI should first have a study phase to look at:

-   -   Limitations of the current mechanisms and measurement quantities        for redistribution of UEs amongst multiple LTE carriers.

Based on the analysis of the study phase, the WI should providesolution(s).

-   -   To redistribute RRC Idle UEs amongst LTE carriers that minimize        the need for load triggered HO or redirection of UE during        connected mode.    -   Carriers with different cell load, bandwidth and capabilities        should be considered.    -   Both homogeneous and heterogeneous deployment scenarios should        be considered.    -   New measurement quantities, e.g. SINR, for better estimation of        user throughput should be evaluated and introduced, if needed

In this additional note, the potential issues in the cell reselectionmechanism for multi-carrier load distribution are identified in supportof the study phase.

(Deployment Scenarios)

FIG. 10 illustrates deployment scenarios. The WID and the motivationdocument point out the examples of multi-carrier deployment scenario,such as homogeneous coverage among multiple carriers (inter-frequencyHomoNet), heterogeneous coverage between multiple carriers(inter-frequency HetNet), and the combination of these two scenarios.Although a unified solution applicable to all three scenarios isdesirable, scenario-specific solutions should also be considered ifsignificant benefits can be realized. Therefore, any proposed solutionshould be evaluated based on its applicability to specific deploymentscenario(s) and whether it causes degradation (e.g., ping-ponging inreselection) in the non-intended deployment scenarios.

The solutions should be applicable to at least one deployment scenarioin inter-frequency HomoNet, inter-frequency HetNet or the combination ofthe two scenarios, and the solutions should not have negative impact onthe non-intended scenarios.

(Limitation in the Existing Cell Reselection Mechanism)

The cell (re)selection parameters are provided by SIBs or dedicatedsignalling (RRC Connection Reject or RRC Connection Release messages)and the cell reselection procedure is performed in the UE using theparameters. Focusing on normal cell reselection (i.e., without dedicatedparameters), SIB3 provides common parameters, while SIB4 providesintra-frequency specific parameters and SIB5 provides inter-frequencyspecific parameters. With these parameters, the UE performs frequencyprioritization according to the cell reselection priority, themeasurements of RSRP/RSRQ, the evaluations of the S-criterion, and theranking of cells with the R-criterion, and then it finds out the mostsuitable cell to reselect and camp on.

In the prioritization and measurement phase, the UE may not performintra-frequency measurement when the S-criterion of the serving cell isfulfilled. In addition, the UE may not perform inter-frequencymeasurement unless the reselection priority of the neighbour frequencyis higher than that of the serving frequency or the S-criterion of theserving cell is not fulfilled. It's obvious that the cell resectionprocedure is not triggered unless the measurement is performed, even ifmore suitable cell is actually available. It's one of limitations in thecurrent cell reselection procedure and causes lopsided UE distributionin some cases, e.g., “lock-on” with idle mode mobility illustrated inFIG. 11.

In addition, there are exceptions in the prioritization phase. The UE isallowed to deviate from the normal frequency prioritization to selectionrule and select a different frequency that provides MBMS services ofinterest. Moreover, Rel-12 ProSe direct communication introduced asimilar rule that “If the UE capable of ProSe direct communication isconfigured to perform ProSe direct communication and can only performthe ProSe direct communication while camping on a frequency, the UE mayconsider that frequency to be the highest priority”. Therefore, it islikely that cells on frequencies offering MBMS and/or ProSe experiencecongestion more often than those that do not, if there are many UEsinterested in these types of services. FIG. 11 illustrates “Lock-on”within higher priority frequency.

Note that it is assumed that the cell reselection for inter-RAT and CSGcell is out of the scope of this work item.

RAN2 should consider how load balance may be achieved between a lowpriority frequency and a high priority frequency with the understandingthat the UE is also allowed to prioritize a frequency based on itsinterest in MBMS and/or ProSe.

In the evaluation and ranking phase, both the S-criterion and theR-criterion use RSRP and RSRQ which are measured by the UE. However, itwas pointed out in that RSRQ is not a sufficient metric for determiningSINR since RSRQ has a narrow dynamic range and it becomes a non-linearfunction of SINR above 5 dB and is saturated around 10 dB. However, thethroughput is kept almost linear between 0 dB and 30 dB. The limitationwith RSRQ measurement is particularly troublesome for higher categoryUEs (e.g., smartphones) since this may degrade the achievable userthroughput. Therefore, in order to maximize achievable user throughputafter transitioning to RRC CONNECTED, at least the ranking process usingRSRQ is no longer adequate and a new measurement metric, with a betterestimate of SINR is desirable.

At least the ranking process for the higher category UEs should be basedon a more accurate measurement metric than RSRQ.

Unlike smartphones, the MTC devices with lower UE category don't needtoo much higher SINR, since the throughput ends up hitting the limitscapped by the Maximum number of DL-SCH transport block bits receivedwithin a TTI. Especially for such lower category UEs, it should be takeninto consideration to avoid unnecessary power consumption due to theincreased measurements using the new metric, as stated in the WID. RAN2should avoid additional power consumption for lower category UEs.

(Potential Issues and Challenges)

In this section, the potential issues, other than the limitationsidentified in section 2.2, are discussed.

(Potential Capacity (Static)/Cell Load (Dynamic) Awareness Distribution)

The potential capacities of frequencies/cells are different since cellshave independent configuration of parameters such as bandwidth, CPlength, almost blank subframes (ABS), MBSFN subframes, ProSe resourcepools, number of antennas and/or cell sizes depending on transmissionpower of eNB and operating frequency. These can be categorized intofrequency-domain, time-domain and space-domain (illustrated in FIG. 12).FIG. 12 illustrates potential capacity and (semi-) staticconfigurations. The potential capacity of a cell may be determined as afunction of the above parameters, and since these configurations aretypically (semi-) static and are already provided in SIBs, they may beacquired by IDLE UEs through UE implementation or assistance from theirserving cells. Since a cell's capacity is directly related to thepotential for user throughput UEs should take such cell capacity intoaccount as part of cell reselection.

The cell reselection procedure should take into account theconfigurations of neighbour cells.

As intended in, more dynamic information such as current cell loads willfacilitate more suitable cell reselection; however, the direct broadcastof load information isn't acceptable by some operators based on previousdiscussions e.g., Rel-12 WLAN Interworking. As in the case for WLANInterworking, the cell loads are implicitly provided through theconfigurations of RAN assistance parameters, i.e., RSRP/RSRQ thresholds.Since a cell's load is an integral part of idle mode UE distribution, itshould be considered whether implicit or direct configuration of acell's load should also be considered for optimizing cell reselection

RAN2 should discuss whether it can be acceptable to provide dynamicinformation such as current cell load to IDLE UEs, directly orimplicitly.

(Clustered UE Distribution)

In general, it is well-known that the density of users is not uniform inan area and the user tends to cluster within specific spots, e.g., in abuilding, on a main street, in a stadium and so on. Therefore, thedistribution of UE clusters among multiple carriers within an eNBcoverage area may be one of the keys to successful load distribution.

With connected mode UEs, load balancing may be resolved using existingmechanisms such as redirections or handovers. On the other hand, idlemode UEs basically applies the common configuration provided by SIBs.Assuming the clustered UEs experience similar radio conditions, i.e.,RSRP and RSRQ, the current cell reselection mechanism cannot split thecluster, i.e., these UEs may reselect based on the sameS-criterion/R-criterion. So, even if cell reselection parameters areadjusted by the eNB, it simply results in “mass reselection” and loadbalancing among multiple carriers cannot be achieved (as illustrated inFIGS. 13a and 13b ). FIGS. 13a and 13b illustrate mass reselection ofclustered UE. To avoid the mass reselection and achieve better loadbalancing, it should be possible for a group of UEs within the clusterto select cell(s) that differ from cell(s) selected by other group ofUEs within the cluster. It should be further considered whether it wouldbe beneficial for the cells to be reselected using some kind of UE-by-UErandomization scheme. Such a randomization scheme could be based on anacceptable measurement range e.g., Cell Reselection Priority and/orR-criterion, where the measurement range is under control of the eNB.Therefore, RAN2 should consider how to achieve load balance of theclustered UEs.

RAN2 should consider how to achieve load balancing of the clustered UEsamong multiple carriers.

[Additional Note 2]

(Introduction)

The discussion on the multi-carrier load distribution WI is started andapproved the requirements as follows;

Requirements Targeted in this WI

1) It should be possible under network control to re-distribute amongthe different carries a fraction of users currently camped on thesecarriers

2) It should be possible under network control to distribute among thedifferent carries a fraction of users moving into the cells from othercells

3) Different deployment scenarios should be supported—macro onlynetworks, co-channel and inter-frequency small cell deployments

4) It should be possible to control the load distribution amongindividual cells rather than only on a carrier level (for example thescenario that the macro cell in a co-channel Het-Net deployment and/orcertain small cells on another carrier may be overloaded)

5) Solutions should cater for different (re)distribution decisions inthe network that take into consideration other factors:

a) eMBMS deployments on macro or small cell layer

b) Number of devices supporting certain bands (other capabilities can beconsidered)

c) Bandwidth of the different carriers may be different

6) The solution should avoid a user ping-pong among carriers

7) Maximize user throughput and network capacity (in terms of systemthroughput, connection establishment, RA, (inter-frequency) mobilityrelated signalling) for UEs in CONNECTED.

In this additional note, the benefits and shortcomings in the possiblesolutions with per-cell priorities and/or reselection probabilities areconsidered.

Discussion

(Evaluation Scenario)

As discussed in, it's one of big challenges to solve “mass reselection”scenario due to clustered UE. The scenario is the same with Scenario 2in. With the current cell reselection mechanism, the UEs which arelocated in the same geographical area, i.e., experience similarRSRP/RSRQ, cannot be distributed among different carriers under networkcontrol. The solutions should be expected to solve this issue.

(Possible Solutions)

The five possible solutions were summarized in, and there are twopromising solutions on the agreement that It should be possible tocontrol the load distribution among individual cells rather than only ona carrier level, the cell specific priority (CSP) and the cell specificpriority probability (CSPP).

CSP provides a new cell reselection priority for specific cell inaddition to the existing cellReselectionPriority for each frequency, andthe cell specific priority is applied when the specific cell is rankedthe highest on a frequency. The UE is mandated to performinter-frequency measurement during the cellReselectionPriority isprovided.

CSPP provides a new probability (Prs) for UEs to determine whether itshould perform cell reselection to a different frequency or stay in thecurrent frequency. And if the probability outcome is such that the UEshould select a different frequency then the cell selection priority onis based on the same mechanism as CSP. CSPP will be capable tore-distribute among the different carries a fraction of users currentlycamped on these carriers, thanks to the probability.

The agreement implies that the serving cell provides an inter-frequencypriority list for specific cell(s) based on probability.

(Reselection Priorities Handling Aspects)

As suggested in, CSP and CSPP work well for offloading the idle UEs tothe other frequency/cell.

However, both CSP and CSPP need to configure the reselection prioritywith higher than current frequency and it force the UEs continuousinter-frequency measurement, which results in additional powerconsumption. To minimize UE power consumption, the load re-distributionmechanism is expected to be done within equal priority frequencies.Furthermore, CSP cannot solve the mass-reselection, i.e., all UEs campedon a cell have to move to the other cell, as also pointed out in.

Proposal: The solutions should work within equal priority frequencies,to avoid unnecessary UE power consumption.

(Measurement Rules for Cell Reselection Aspects)

Assuming equal priority frequencies, the current specification definesthat If the serving cell fulfils Srxlev>S_(nonIntraSearchP) andSqual>S_(nonIntraSearchQ), the UE may choose not to perform measurementsof E-UTRAN inter-frequencies or inter-RAT frequency cells of equal orlower priority, which is important rule to reduce UE power consumption.On the other hand, for the purpose of load re-distribution of UEs it maybe necessary to force some UEs to perform inter-frequency measurementand cell reselection as suggested by CSP or CSPP whenever a higherpriority cell is provided in the cell specific priority list, which willresult in additional UE power consumption. So, it's preferable thatinter-frequency measurement and cell reselection should be performedonly once for load re-distribution. For example with CSPP, the givenprobability, Prs=0.2, is applicable for a one-shot reselection in agiven re-distribution event, i.e., if the cell reselections areperformed twice or more then Prs will no longer be 0.2, as shown in FIG.14. FIG. 14 illustrates number of UEs moved to other frequency vs. # ofcell reselections.

Proposal: The UE should perform inter-frequency measurement and cellreselection only once in a given load re-distribution event.

To avoid multiple inter-frequency measurements and cell reselections,the re-distribution of UEs should be triggered by such as “reselectionindication” provided in SIB or dedicated signalling. The “reselectionindication” also includes a counter value, as similar to the existingvalue tag in SIB, in order that the UE identify whether theinter-frequency measurement and cell reselection are performed by the“reselection indication”. The counter value is assumed to be increasedwhen a cell reselection is triggered. The UE would compare the currentcounter value to the latest counter value which previously triggered acell reselection, and it determine whether a new cell reselection shouldbe performed or not.

Proposal: The inter-frequency measurement and cell reselection should betriggered by an indication provided in SIB or dedicated signalling,which possibly contains a counter value in order to confirm validity ofthe indication.

(Cell Reselection Criteria Aspects)

According to the current specification, intra-frequency cells and equalpriority inter-frequency cells are ranked by R-criteria, and the bestcell in the ranking is selected as the new cell. The rule ensures thebest performance in UE throughput when the UE transitions to Connectedfrom RSRP point of view, although it cannot consider RSRQ/SINR and cellload aspects. It cannot fulfil the agreed requirement that Maximize userthroughput and network capacity (in terms of system throughput,connection establishment, RA, (inter-frequency) mobility relatedsignalling) for UEs in CONNECTED.

Propsal: The solutions should consider enhancements in the rankingmechanism to maximize user throughput and network capacity.

CSPP does not use this rule since the UE will randomly select a cellamong the same priority cells, while CSP reuses the rule. Although thisrule is beneficial in the past, it should be enhanced for today'smulti-carrier operations, i.e., to avoid the mass reselection. It couldbe considered there are four options as follows;

Option 1: Ranking Randomization (RR), as is in CSPP.

In contrast to the current ranking mechanism, the cell is reselectedrandomly from all cells which fulfil S-criteria, i.e., not only the bestranked cell but also the second ranked cell, the third ranked cell andso on. This option can solve the mass reselection problem, although it'spossible to reselect worse cell than expected from the radio conditionpoint of view, and it results in increasing handover for throughputoptimization when the UE transitions to Connected.

Option 2: Ranking Randomization with Range (RRR)

This option is enhancement over option 1 (RR). As same as option 1, thereselected cell is randomized regardless of its ranking, but it'slimited within a range. The range is set with e.g., 3 ranks, and thenthe cell is reselected from top three cells in the ranking, the bestcell, the second best cell or the third cell. The rank may be enhancedwith value of X dB, and then the cell is chosen from the cells fulfilits RSRP is over RSRP of the best ranked cell minus X dB. The benefit ofthis option over option 1 (RR) is to ensure moderate signal strengthwithin the range, which contributes to reduce handover for userthroughput optimization when the UE transitions to Connected.

Option 3: Ranking Specific Probability (RSP)

With this option, probabilities for each rank are provided by theserving cell. For example with three ranks, the probabilities are setwith 50% for the best rank, 30% with the second rank and 20% for thethird rank. The UEs uses random number in the ranking mechanism and 50%of the UEs reselect the best ranked cell. In similar fashion, 30% of theUEs reselect the second best cell and 20% reselect the third rankedcell. The benefits in this option is not only to build on the existingranking mechanism but also to avoid reselecting the worse ranked cell,e.g., the fourth ranked cell which reduce handover for user throughputoptimization when the UE transitions to Connected.

Option 4: Grouping and Group Specific Re-Distribution (GSR)

This option assumes that a dedicated signalling, e.g.,RRCConnectionRelease, includes specific group ID. After the grouping,the serving cell may provide in SIB different set of cell reselectionparameters and/or a trigger of cell reselection for each group.Considering all UEs transition to Connected during e.g., initial attach,it could be possible to configure all UEs with each group. It's FFS howto handle the group when UEs perform cell reselection.

Option 1 cannot avoid handover for user throughput optimization when theUE transitions to Connected, while the other options can solve it.Option 2 and Option 3 can perform “soft” re-distribution among cellsunder network control. Option 4 can work as “full network controlled”re-distribution. Considering complexity of the control, option 2 oroption 3 is slightly preferable solution.

Proposal: RAN2 should consider either the ranking randomization with arange (RRR) or the ranking specific probability (RSP) for equal priorityinter-frequency cell reselection.

[Additional Note 3]

This additional note considers the solutions with per-cell parameterand/or reselection probabilities for the one-shot re-distributionmechanism.

(One-Shot Re-Distribution Mechanism)

The one-shot re-distribution obviously needs a trigger for theinitiation of special cell reselection procedure. The followingsolutions for cell reselection by fraction of UEs are considered afterthe triggering.

(Cell Reselection by Fraction of UEs)

It's one of big challenges in this WI that Solution should be able tomove fraction of the UEs from one cell to another cell. The possiblesolutions were suggested as follows;

Assuming the cell-specific priority list is provisioned in advance andthe priority has to be higher than that of the serving cell;

Option 1-a: Cell-Specific Priority with Random Threshold Offset

If it could be applicable to the one-shot re-distribution, when atrigger happens the UE generates the random value for a cell of higherpriority frequency, and the UE reselects the cell ifSqual>Thresh′_(X, HighQ) or Srxlev>Thresh′_(X, HighP) is fulfilled,wherein Thresh′_(X, HighQ) or Thresh′_(X, HighP) is applied with therandomized offset (i.e.,Thresh′_(X, HighQ)=Thresh_(X, HighQ)+offset_(x/cell)*random).

One drawback with this approach is that the Thresh′_(X) may not ensuregood user throughput with wider/negative range of the randomized offsetand that only UEs in cell-edge can move to the other cell with narrowerrange of the randomized offset.

Option 1-b: Cell-Specific Priority with Probability (CSPP)

If it could be applicable to the one-shot re-distribution, when atrigger happens the UE generates the random value for the cellprioritized and reselect it if the random value exceeds the probabilityprovided.

One drawback may be that the cell reselected can only ensure S-criteria,not for Thresh_(X) nor R-criteria. It may result in the degradation ofuser throughput when the UE transitions to RRC Connected.

Option 1-c: Counter-Based Scheme with Number of Cell Reselection

If it could be applicable to the one-shot re-distribution, whereby theUE keeps track of the number of reselections and when a trigger happens,the UE decides whether the cell reselection priority should be appliedor ignored based on the counter value.

Independent of the priority handling, i.e., not only for higher prioritycells;

Option 2-a: Ranking Randomization within a Specified Range

This option is a kind of extension on Option 1-b (from the randomizationpoint of view) and essentially similar approach with Option 1-a (fromthe range concept perspective), but Option 2-a focuses on theenhancements in the ranking process. With the specified range, thedegradation of user throughput may be limited and is under networkcontrol. The details of range are FFS, e.g., the unit may be [dB].

Option 2-b: Ranking-Specific Probability

This option is basically similar approach with Option 2-a, but Option2-b focuses on the enhancements in the ranking process withprobabilities similar to Option 1-b. Since the probability iscorresponding to each rank, the UE is allowed to reselect the cell withbetter rank order and probabilities.

All options could move fraction of the UEs from one cell to anothercell. The most significant difference between Options 1-x and Options2-x is whether a cell specific priority list (CSP) is needed or not.Since Options 1-x would only take the priority handling in thereselection consideration, the reselected cell may not be (sub-)optimalfor each UE from the radio quality point of view, i.e., the new cell mayonly fulfil the S-criterion even if some cells offering better radioquality are available around the UE. In addition, Options 1-x cannotconsider equal priority frequencies/cells since these options rely onthe configuration with higher priority frequencies/cells.

Options 2-x may be based on the existing ranking process which ensured acell reselection to a better cell as the result of comparison to cellsthe UE detects. So, Options 2-x can potentially Maximize user throughputand network capacity (in terms of system throughput, connectionestablishment, RA, (inter-frequency) mobility related signalling) forUEs in CONNECTED. Therefore, the one-shot re-distribution should adaptthe ranking process.

Proposal 1: The ranking process should be enhanced for the one-shotre-distribution.

(Target Cells/Frequencies for Enhanced Ranking Process)

If Proposal 1 is agreeable, the issue is which cell/frequency should beconsidered in the enhanced ranking process. In the currentspecification, the evaluation with the R-criteria applies forintra-frequency and equal priority inter-frequency cells. So, tominimize changes in the specification, the target cells/frequencies forthe enhanced ranking process should also be considered under equalpriority cells/frequencies.

Proposal 2: The target cells/frequencies in the one-shot re-distributionshould only be applicable under equal priority.

If Proposal 2 is acceptable, it's necessary to determine how the equalpriority target cells/frequencies are provided to the UE for theone-shot distribution. The following alternatives could be considered;

Alt.1: The serving cell provides the cell/frequency list explicitly;

The UE simply consider the cells/frequencies provided in the list in theenhanced ranking process.

Alt.2: The serving cell provides the cell/frequency implicitly;

It could use the extended cell reselection priority. For eachcell/frequency configured with the extended cell reselection priority,the UE determines these cells/frequencies for enhanced ranking process.It's FFS whether the UE should also take the legacy cell reselectionpriority into account or not, while some examples are suggested on Table1.

From the perspective of signalling overhead, Alt.2 is better than Alt.1.However, Alt.2 will need at least one more rule to determine the targetcells/frequencies, e.g., whether the prioritized cells/frequencies inthe legacy cell reselection priority should still be prioritized.Although pros and cons can be seen in both alternatives, Alt.2 isslightly preferred solution.

Proposal 3: RAN2 should decide if the cells/frequencies configured withthe extended cell reselection priority should be considered as equalpriority in the one-shot re-distribution.

TABLE 1 Examples of implicit target cells/frequencies provisioning withAlt. 2 Legacy priority i.e., Extended priority e.g.,cellReselectionPriority CellReselSubPriority-r12 [10] [11] Alt. 2-1 Alt.2-2 Alt. 2-3 Alt. 2-4 F1 7 3 Target Target Target Target F2 7 3 TargetTarget Target Target F3 7 1 Target Target F4 7 (not provided) F5 3 3Target Target F6 3 1 Target F7 3 (not provided) Note: The serving cellmay be legacy priority 5, and the target cells/frequencies areconfigured; Alt. 2-1: With a (e.g., higher) legacy priority and anyextended priority. Alt. 2-2: With a (e.g., higher) legacy priority and a(e.g., higher) extended priority. Alt. 2-3: With any extended priority,regardless of the legacy priority. Alt. 2-4: With a (e.g., higher)extended priority, regardless of the legacy priority.

(Additional Considerations)

Further Considerations for Randomization

It was pointed out that the probability-based scheme has some concern onthe testability and/or controllability of the reselection by thenetwork. Thus, the counter-based scheme was suggested as an alternativeto the probabilistic-based scheme. However, if the probability-basedscheme is really problematic, it could also be considered whether therandomization may be based on the UE_ID (or the IMSI) similar to theexisting paging frame/occasion determination. For example of IMSI, theUE could perform the (special) cell reselection procedure if (IMSI modNp)=Nr is fulfilled, wherein Np and Nr configured based on the expectedprobability and fairness among UEs. If Np and Nr are configured with 10and 0 respectively, the probability fulfilling the formula is 10%. Ifthe IMSIs are assumed adequately randomized among UEs within a cell, thenetwork may control the probability, while the UE testability would bemuch simpler.

Proposal 4: If the probability-based scheme is not preferred, theIMSI-based randomization should be one of potential methods instead.

Possibility of RSRQ/SINR-Based Ranking Process

The current R-criteria only consider RSRP, thus any radio quality arenot taken into account. If only received power is evaluated, it couldensure the optimal throughput in homogeneous network with macro cells.However, today it cannot be true with HetNet deployments, e.g., thesmall cell located near a macro cell may offer better RSRP but worseRSRQ than a cell on the frequency layer with small cell only. Tomaximize the user throughput and network capacity when the UEtransitions to Connected, it's worth revisiting the R-criteria with RSRQor SINR. If IDLE UEs uses a more accurate indication of signal qualitythan what is used today, it would potentially meet the requirement toMaximize user throughput and network capacity (in terms of systemthroughput, connection establishment, RA, (inter-frequency) mobilityrelated signalling) for UEs in CONNECTED.

Proposal 5: RAN2 should consider whether the R-criteria should berevisited using a different signal quality criteria considering HetNetdeployments.

[Additional Note 4]

(Introduction)

In this additional note, the two mechanisms are identified for themulti-carrier load distribution, i.e., the continuous distribution andthe one-shot re-distribution, and the solutions with per-cell parameterand/or reselection probabilities are considered.

(Pre-Conditions and Post-Conditions)

Before considering the solutions, it's worth revisiting why the idle UEsmay be distributed non-uniformly among multiple carriers today. FIG. 1illustrates three possible pre-conditions, i.e., (a) to (c), and apost-condition, i.e., (d), assuming four frequency layers for thepurpose of coverage and capacity. F1 is the lowest frequency (e.g., 800MHz, thus it has larger coverage) and F4 is the highest frequency (e.g.,3.5 GHz, thus it has smaller coverage), wherein it's noted in FIG. 1that the condition (b) has reverse order compared to the otherconditions. In the condition (a) and (b) each frequency layer hasdifferent cellReselectionPriority, while the condition (c) intends equalpriority among all frequencies.

The condition (a) may be considered as a typical priority configuration,whereby the small cell layers on higher frequencies are prioritized forreselection to achieve capacity enhancements. The UEs are camped on F4or F2 as long as the S-criteria of the cell is fulfilled, since thecurrent specification forces the UE to perform inter-frequencymeasurements and cell reselection to a cell configured with higherpriority frequency than the frequency of the serving cell, regardless ofSqual/Srxlev of the serving cell.

The condition (b) may be considered for the coverage-prioritizedconfiguration e.g., in Dual Connectivity capable network. This resultsin a worse condition than condition (a), since all UEs are camped on F1.

It should be noted that the cellReselectionPriority may cause loadimbalance of idle UEs beyond just two frequency layers, i.e., imbalancebetween F1 and F2, or between F3 and F4 in FIGS. 15a to 15d . Althoughthe current mechanism allows configuring differentThresh_(X, HighQ)/Thresh_(X, HighP) for each frequency priority, itcannot solve the distribution issue since a change of these parameterscause mass reselection and agreement 2 “Solution should be able to movefraction of the UEs from one cell to another cell” above cannot be met.Even if the cell specific priority is provided in Rel-13, theprioritized cell may experience higher load due to the mass reselection(also known as “surge of UEs” or “closely located UEs problem”) assumingthe cells offers the same capacity (e.g., the same bandwidth), althoughthe solution may work well in a specific scenario, e.g., reselection tosmall cells on co-channel HetNet layer.

The cell reselection prioritization itself may cause load imbalanceamong multiple frequencies, regardless of whether it's on a frequencylayer or a cell layer.

Condition (c) configures all frequency layers with the same priority,i.e., equal priority inter-frequency case. With equal priorityinter-frequency, the cell ranking is performed by R-criteria and the UEshould select a cell with the best RSRP, taking into account of theQoffset to compensate for different pathloss due to differentfrequencies. As in the case for conditions (a) and (b), the currentmechanism also cannot meet the agreement that Solution should be able tomove fraction of the UEs from one cell to another cell. However, thecurrent cell ranking mechanism is useful in solving the issue “Maximizeuser throughput and network capacity (in terms of system throughput,connection establishment, RA, (inter-frequency) mobility relatedsignalling) for UEs in CONNECTED”, since the UE can reselect the cellthat offers the best radio condition, although it only relies on RSRP.

The equal frequency priority with cell ranking has a potential tomaximize user throughput and network capacity.

The condition (d) is an example of a desirable post-condition, whereinthe UEs are camped on each frequency/cell uniformly and all cells arenot heavy-loaded. The post-condition in (d) is consistent with theagreement above that a Solution should be able to move fraction of theUEs from one cell to another cell which may be using e.g. per-cellparameter and/or reselection probabilities.

The most significant benefit of the enhanced continuous distributionmechanism such as CSP is to allow the cells to remain lightly-loaded,i.e., the network's UE distribution may be maintained and used toprevent the pre-conditions (a)˜(c) in FIGS. 15a to 15d from occurring.On the other hand, one of the main drawbacks with the enhancedcontinuous distribution is the impact to the power consumption of IDLEUEs even though the network does not suffer from any over-loadconditions. If further solutions such as the cell-specific priority withprobability and/or with random threshold offset are introduced, the UEpower consumption may be substantially increased. Additionally,operators may face challenges to modify their existing networkreselection mechanism even if the operator does not suffer from thenecessity redistribute idle UEs.

The enhanced continuous distribution mechanism is expected to have moreimpacts on the UE power consumption and the configurations of theexisting network deployments.

In light of Observation 3 if an enhanced continuous distribution isintroduced to fulfil the agreement that 4) It should be possible tocontrol the load distribution among individual cells rather than only ona carrier level (for example the scenario that the macro cell in aco-channel Het-Net deployment and/or certain small cells on anothercarrier may be overloaded), it should not require drastic changes tonetwork deployment, but simply aim to provide more flexibilities basedon the existing mechanism.

Proposal 1: The existing reselection mechanism or the small enhancementmay work well, other than some heavily loaded network deploymentscenarios.

Even if the enhanced continuous distribution could statisticallymaintain the network under light load, the traffic demands, which iscorrelated with the density of IDLE UEs in an area, could varysignificantly e.g., at sports events, during commuting hours, within ashopping mall in weekends or under emergency conditions. In such aheavy-loaded condition, there should be a means for the network toreturn to a more balanced loaded condition as soon as possible, by meansof a one-shot re-distribution mechanism. Once the one-shotre-distribution mechanism allows the network to return to a normal loadcondition, the enhanced continuous distribution may be activated againto maintain the network under the balanced load condition.

The one-shot re-distribution mechanism may be useful under heavilyloaded network due to sudden surge in traffic demand.

Since the one-shot re-distribution mechanisms is mainly used for casesof heavily loaded network due to sudden surge in traffic demand, it maybe applied independently of the existing mechanisms and thus it will notconflict with the current network strategies. Furthermore, the impactson UE power consumption would be reduced since the power-consumed eventis performed only once. The network may initiate the one-shotre-distribution when it experiences higher load in multiplecells/frequencies, i.e., the preconditions (a)˜(c) in FIGS. 15a to 15d .In addition, the word “re-distribute” within agreement 1) above isconsistent with the need for a one-shot re-distribution mechanism, sincethe continuous distribution or enhanced continuous distribution shouldnot be depicted by as a means to “re-distribute” the UEs.

Proposal 2: RAN2 should consider the one-shot re-distribution mechanismas an independent mechanism from the continuous distribution or enhancedcontinuous distribution mechanism.

(Enhanced Continuous Distribution Mechanism)

For the extreme case when the network is lightly loaded and there isonly one UE camped on the macro cell, there is no reason for the UE tostay on the macro cell layer when a small cell fulfils a threshold forthe UE. In this case, either the prioritization mechanisms, or theexisting frequency priority or the additional cell-specific priority(CSP), still works well. With the CSP which was already a baseline, thenetwork is offered more flexibility in its configuration with minimumimpacts on the existing rules for the priority handling. This is similarapproach with the agreements for IncMon to Extend the number of cellreselection priorities to reduce number of reselections between equalpriority carriers, from the configuration flexibility point of view.Therefore, the enhanced continuous distribution based on the existingcell reselection mechanism should only have an additional means toconfigure priorities on a cell basis with CSP, although it should benoted that it potentially has a drawback which may lead to loadimbalance when the number of UEs increases, as suggested in Observation1, in which case another solution may be needed, i.e., the one-shotre-distribution mechanism.

Proposal 3: RAN2 should conclude for lightly/medium loaded network thatno additional mechanism is necessary in the existing cell reselectionmechanism, except for optional provisioning of the cell-specificpriority and the already agreed extended reselection priorities.

(Benefit of One-Shot Re-Distribution Mechanism)

Trigger for Re-Distribution

The trigger for re-distribution, i.e., when the UE initiates/continuesthe (special) cell reselection procedure, may be based on one of thefollowing options:

Option 1: When the network broadcasts parameters for there-distribution;

Option 1-a: When the network provides the list of cell-specificpriorities;

It's assumed as the trigger for the cell-specific priority withrandomization as well as CSP itself. The UE should continue the cellreselection procedure, including inter-frequency measurements, as longas the cell-specific priority provided is higher than the priority ofserving cell, as it is today.

Option 1-b: When the network provides the list of cell-specificprobabilities;

This option assumes the trigger for reselection is based on thecell-specific priority with probability (CSPP). The UE should generate arandom value and perform the cell reselection procedure when the randomvalue is over the cell-specific priority provided.

Option 1-c: When the network provides the maximum value of reselections;

This option assumes the counter-based scheme. The UE should count thenumber of cell reselection performed and ignore the higher-prioritizedcell when the count is over the maximum value broadcasted by the servingcell.

Option 1-d: When the network updates the parameters;

It's assumed the trigger is implied by the updated parameters in SIB orthe paging. The UE should apply the updated parameters and perform thecell reselection procedure.

Option 2: When the network provides an explicit request;

This option assume a pure trigger for re-distribution. The UE appliesthe (special) parameters and perform the cell reselection procedure onlyonce upon reception of the request. The request may require 1-bitsignalling in either a SIB or in the paging message as an application.The UE may only monitor the request since the parameter for the(special) cell reselection can be broadcasted in advance.

Option 3: When the serving cell configures dedicated parameters over RRCConnection Release;

It's for the specific case the UE transitions from RRC Connected to Idleand the RRC Connection Release may have the additional cell-specificoffsets, the extra grouping, and/or the existing dedicated priority.

Options 1-x are based on typical mechanisms for reselection control, butthese have some drawbacks. With Options 1-x, the UE is forced to applyadditional continuous inter-frequency measurements during the parametersare provided, e.g., if the cell prioritized by Option 1-a is not on thefrequencies prioritized by the existing cellReselectionPriority. Itresults in additional UE power consumption and it should be avoided.Also, it's not crystal clear at which points in time the UE shouldre-calculate the probability.

Additionally, Options 1-x may require the UE to monitor continuously tocheck whether the parameters are updated, or whether the network decidedto initiate a (special) cell reselection procedure. Option 2 is a simplesolution for the one-shot re-distribution mechanism, although it doesrequire 1-bit signalling to inform UE of the activation of thisprocedure. Option 3 may also be an nice way if the solution only relieson the dedicated signalling, but it cannot fulfil the requirement toMaximize user throughput and network capacity (in terms of systemthroughput, connection establishment, RA, (inter-frequency) mobilityrelated signalling) for UEs in CONNECTED. Therefore, Option 2 is thepreferred solution for the trigger for re-distribution.

Proposal 4: The one-shot re-distribution should be triggered by anexplicit request indication broadcasted by the serving cell.

Proposal 5: If Proposal 4 is agreeable, RAN2 should discuss whether theindication is provided in SIB or the paging.

The invention claimed is:
 1. A user equipment for a mobile communicationsystem, comprising: a receiver configured to receive a first parameterbroadcasted from a current serving cell and a second parameterbroadcasted from the current serving cell; and a controller configuredto reselect a target cell to be used as a serving cell from among aplurality of cells operated at different frequencies, wherein the firstparameter is a parameter that defines a probability for performingreselection of the target cell, the second parameter is a timer value tobe set in a timer started at a timing at which a quality measurement forneighbor cells ends, and the controller is configured to after receivingthe first parameter and the second parameter, periodically measurequalities of neighbor cells when the timer expires, compare a valuecalculated by modulo operation using IMSI (International MobileSubscriber Identity) of the user equipment with a value corresponding tothe first parameter, and reselect the target cell from among cellshaving a quality satisfying predetermined quality criteria, based onresults of the quality measurement and the comparison.
 2. A methodperformed at a user equipment for a mobile communication system,comprising: receiving a first parameter broadcasted from a currentserving cell and a second parameter broadcasted from the current servingcell; and reselecting a target cell to be used as a serving cell fromamong a plurality of cells operated at different frequencies, whereinthe first parameter is a parameter that defines a probability forperforming reselection of the target cell, the second parameter is atimer value to be set in a timer started at a timing at which a qualitymeasurement for neighbor cells ends, and the reselecting comprises:after receiving the first parameter and the second parameter,periodically measuring qualities of neighbor cells when the timerexpires; comparing a value calculated by modulo operation using IMSI(International Mobile Subscriber Identity) of the user equipment with avalue corresponding to the first parameter; and reselecting the targetcell from among cells having a quality satisfying predetermined qualitycriteria, based on results of the quality measurement and thecomparison.
 3. A chip for a user equipment for a mobile communicationsystem, comprising: at least one processor and at least one memory, theat least one processor configured to perform processes of: receiving afirst parameter broadcasted from a current serving cell and a secondparameter broadcasted from the current serving cell; and reselecting atarget cell to be used as a serving cell from among a plurality of cellsoperated at different frequencies, wherein the first parameter is aparameter that defines a probability for performing reselection of thetarget cell, the second parameter is a timer value to be set in a timerstarted at a timing at which a quality measurement for neighbor cellsends, and the reselecting comprises: after receiving the first parameterand the second parameter, periodically measuring qualities of neighborcells when the timer expires; comparing a value calculated by modulooperation using IMSI (International Mobile Subscriber Identity) of theuser equipment with a value corresponding to the first parameter; andreselecting the target cell from among cells having a quality satisfyingpredetermined quality criteria, based on results of the qualitymeasurement and the comparison.